Design of fluorescent metal sensors has recently become one of the most active research areas because the sensors can provide in situ and real-time information for a number of applications including environment monitoring, industrial process control, metalloneurochemistry, and biomedical diagnostics.1 A widely used strategy is to link the metal recognition portion closely with a signal generation moiety such as a fluorophore. While quite successful in designing sensors for diamagnetic metal ions such as Pb2+, Hg2+, Zn2+ and Cu+,2 this method has been applied to paramagnetic metal ions such as Cu2+ with only limited success, due to their intrinsic fluorescence quenching properties.3,4 Most Cu2+ sensors showed decreased emission upon Cu2+ binding,3 which was undesirable for analytical purposes. First, the room for signal change was at most one-fold. Second, such “turn-off” sensors may give false positive results by quenchers in real samples. Among the reported “turn-on” Cu2+ sensors,4 few have nanomolar sensitivity,4a,d,f,g with high selectivity,4a,d and are free of organic solvents.4a 
One way to circumvent this quenching problem is to spatially separate the metal recognition part from the fluorescent signaling moiety so that they are independent of each other. A significant challenge then is to transduce metal binding to signal enhancement when the two parts are well separated. Previously reported was a novel metal sensing platform with DNAzyme catalytic beacons that spatially separated the two parts by rigid double-stranded DNA,5,6 and sensors for diamagnetic metal ions such as Pb2+ and UO22+ have been demonstrated.7,8 
Copper is a widely used metal that can leak into the environment through various routes. Low concentration copper is an essential nutrient. However, exposure to high level of copper even for a short period of time can cause gastrointestinal disturbance; while long term exposure causes liver or kidney damage.9 The U.S. Environmental Protection Agency (EPA) set the limit of copper in drinking water to be 1.3 ppm (˜20 μM).